Herbal tea: Yes, for a couple of brewing processes it is possible to make kombucha completely without real tea. It should be noted that the SCOBY is always a little affected by the nutrients of herbal teas, which are usually antibiotic. Therefore, brewing ONLY with herbal tea should not last more than one or two brewing processes. To provide the SCOBY with all the nutrients and still get the benefits of the herbal teas, it is also possible to make a blend with black or green tea.
In any case, you should avoid herbal teas that have too many essential oils and artificial flavoured chemicals. These can harm your SCOBY. Herbs with natural oils such as peppermint and sage should not be fermented with the first fermentation. The SCOBY will in the long run lose the ability to absorb the oxygen it needs to survive.
When you use natural plant materials, you also get all the benefits of the nutrients they bring. The SCOBY will absorb these nutrients and pre-digest them for you. This ensures a higher bioavailability of these nutrients.
Fresh herbs: it is better to dry the herbs beforehand. Then, in any case, boil the tea briefly for 1 minute so that the kombucha is not contaminated!